Fiber-reinforced resin materials, or “composite” materials as they are commonly known, are frequently used for aerospace, automotive and marine applications because of high strength-to-weight ratios, corrosion resistance, and other favorable properties. Conventional composite materials typically include glass, carbon, or polyarymide fiber “plies” in woven and/or non-woven configurations. The fiber plies can be manufactured into composite parts by laminating them together with an uncured matrix material (e.g., an epoxy resin). The laminate can then be cured with the application of heat and/or pressure to form the finished part.
Composite parts can be manufactured from “prepreg” materials, or from dry fiber plies assembled into a “preform.” Prepreg is ready-to-mold material in a cloth, mat, roving, tape or other form that has been pre-impregnated with matrix material (e.g., epoxy resin) and stored for use in an uncured or semi-cured state. The prepreg sheets are laid-up on the mold surface in the shape of the finished part. Pressure is then applied to compact the prepreg sheets, and heat can be applied to complete the curing cycle. A preform is different from a prepreg assembly in that a preform is an assembly of dry fabric and/or fibers which have been prepared for a resin infusion process on the mold surface. The preform plies are usually tacked and/or stitched together or otherwise stabilized to maintain their shape before and during final processing. Once the preform has been stabilized, the layers can be infused with resin using a liquid-molding process. The part can then be cured with the addition of pressure and/or heat.
The fiber material in composite parts provides relatively high strength in the direction of the fibers. Impact resistance, however, is generally determined by the properties of the cured matrix. One way to enhance impact resistance is to add particles of, e.g., a thermoplastic material to the matrix. The thermoplastic material can inhibit crack propagation through the part resulting from, for example, foreign-object debris, which is typically not visible to the naked eye.
Another way to increase the impact resistance and fracture toughness of composite parts is to enhance the structural properties of the bond-line between alternating layers of composite materials (i.e., the interlayer properties). Adding carbon nanotubes to the interlayer is one method for improving the interlayer properties of composite materials. Carbon nanotubes are ordered molecules of pure carbon which form very small cylinders (on the order of 10 nanometers (i.e., 1×10−8 meters)). Carbon nanotubes exhibit unusual strength, and may be over 30 times as strong as typical carbon fibers and 100 times stronger than steel of equivalent weight.
One method for introducing carbon nanotubes in between two fiber plies is to add the nanotubes to the bond-line resin. One shortcoming of this approach, however, is that it is often difficult to maintain an even distribution of nanotubes in the liquid resin. Another shortcoming with this approach is that suspending the nanotubes in resin results in random orientation of the nanotubes between adjacent fiber plies. Moreover, the addition of even small amounts of nanotubes to a liquid resin tends to dramatically increase its viscosity and, thus, decrease its processability. Accordingly, it would be advantageous to have a method for evenly distributing carbon nanotubes in the proper orientation between fiber plies in the manufacturer of composite parts.